Global warming will increase suicides, researchers say

A man takes a break in the shade of a tree at Lake Merritt in Oakland.

Photo: Gabrielle Lurie / The Chronicle

3 of 8

Friends Gabriel (left) and Nick chat in the shade at Lake Merritt.

Photo: Gabrielle Lurie / The Chronicle

4 of 8

A pedestrian looks out onto the Golden Gate Bridge from Torpedo Wharf in San Francisco.

Photo: Liz Moughon / The Chronicle

5 of 8

A man relaxes at Crown Memorial State Beach in Alameda.

Photo: Gabrielle Lurie / The Chronicle

6 of 8

A pedestrian walks on Torpedo Wharf in San Francisco.

Photo: Liz Moughon / The Chronicle

7 of 8

A man walks around Lake Merritt.

Photo: Gabrielle Lurie / The Chronicle

8 of 8

A lady walks in the water at Baker Beach in San Francisco.

Photo: Liz Moughon / The Chronicle

More people are likely to take their own lives as the planet warms, say researchers at Stanford University and UC Berkeley in a study published Monday that suggests yet another worrisome impact of climate change.

The multidisciplinary research team looked at nearly 1 million suicides in North America and found that hotter temperatures correlate with higher suicide rates. The warming projected through 2050, the group figures, could increase suicide rates by 1.4 percent in the U.S. and 2.3 percent in Mexico over that time, resulting in 21,000 additional deaths in the two nations.

The role of heat, the authors said, may be just as significant as other, more well-known drivers of suicide, like economic hardship, which also pushes rates up, and suicide prevention programs and gun control legislation, which tend to push rates down.

“The overall health burden of suicides and poor mental health is already large in this country and it’s going to grow,” said Marshall Burke, the study’s lead author and an assistant professor of earth system science at Stanford.

While earlier research has suggested a link between climate change and suicide, specifically climate-related disasters like drought, the driver was generally thought to be something deeper, like anguish over a financial loss.

Burke and his colleagues, however, said warm weather itself probably has a direct effect on a person’s mental well-being.

The researchers said they evaluated such a large number of suicides over such a long period of time — 866,000 in the U.S. and 74,000 in Mexico as far back as 1968 — that they concluded that other factors couldn’t account for all of the incidents and trends.

“The only explanation is that it’s some sort of underlying biological response to hotter temperatures,” Burke said, noting that some had long suspected the contrary, that cooler temperatures were more problematic. “We had typically thought of people being unhappy in the cold and dark.”

The study indicates that the link between heat and suicides was equally discernible in warm and cold climates.

Since the group’s statistical analysis doesn’t prove the presence of a physiological factor, merely suggests one, the researchers turned to social media to test whether hotter temperatures truly drive a person’s mental state.

“We wanted to be as sure as we could about what we were saying,” said Solomon Hsiang, study co-author and associate professor of public policy at UC Berkeley.

The researchers observed more than 600 million Twitter posts to evaluate moods in different kinds of weather. They found that every 1.8-degree Fahrenheit increase in temperature, or 1 degree Celsius, raises the likelihood of a depressive tweet, one that contains such words as “lonely” or “suicide,” by as much as 1 percent.

The correlation between temperature rise and the pattern researchers saw in tweets is almost exactly the same as that between hot weather and suicide rates, Hsiang said.

“It looks very much like it’s something that’s mental and less contextual,” he said.

The researchers also found that the link between temperature and both depressive tweets and suicide risk is uniform for rich and poor and for men and women.

“Often when you hear climate change discussed in the media or even in the academic literature, you hear that there’s going to be winners and losers,” Burke said. “In suicide, that’s not what we find at all. Everyone is losing.”

Nationwide, 45,000 suicides occurred in 2016, making it one of the top 10 causes of death in the U.S., according to the Centers for Disease Control and Prevention. And it’s a problem that’s on the rise.

In California, suicides have increased 14.8 percent between 1999 and 2016, according to the CDC.

San Francisco Suicide Prevention, which provides counseling and outreach in the city and runs a hotline for those struggling emotionally, reports an uptick in the number of calls over the past year.

Fletcher Johnson, outreach coordinator for the organization, said the bottom line is that people’s emotional needs aren’t being met, with higher temperatures or no higher temperatures.

“There might be a relationship between climate change and suicide,” he surmised, “but it might be simpler than that.”

The new study on suicides is published in the science journal Nature Climate Change.

Kurtis Alexander is a general assignment reporter for The San Francisco Chronicle, frequently writing about water, wildfire, climate and the American West. His recent work has focused on the impacts of drought, the widening rural-urban divide and state and federal environmental policy.

Before joining the Chronicle, Alexander worked as a freelance writer and as a staff reporter for several media organizations, including The Fresno Bee and Bay Area News Group, writing about government, politics and the environment.